The present invention relates to packet-switched data communication systems, and particularly to such systems operating according to the IEEE 802.1 transmission standard.
Data communication systems of this general character provide for communication between various parts of a network by means of physical devices that in general comprise a multiplicity of ports and direct received packets from one or more to one or more ports either indiscriminately, in the case of a hub, or selectively, in the case of a switch. The term “switch” is used herein to embrace more specifically named devices, such as bridges, routers or devices that combine both bridging and routing functions.
In a variety of circumstances it is desirable to be able to couple a plurality of devices, such as a multiplicity of hubs or switches, together so that from the point of view of the external network, the plurality of coupled or “stacked” devices acts as a single logical entity. Such a facility is useful to enable a network to be increased in traffic carrying capacity.
To facilitate the operation of the units of a stack as a single logical entity, it is desirable for a packet progressing through the stack to carry selected information which enables a device within a stack to recognise the data packet as coming from another unit in the stack, or, analogously, from another chip in a device which is composed of a multiplicity of chips connected by links or buses to form a single switch or other network device.
Although it is known generally for devices to identify their nature and status to other devices by means of for example, a “handshake routine”, such routines or other processes which necessarily involve the generation of additional data packets on a link are in general undesirable, particularly at very high data rates. It is accordingly preferable to be able to convey selected or inter-device data within an existing data packet. Theoretically it is possible to append data onto the beginning or end of an existing packet's data field. Such an expedient is not desirable because data packets in general and Ethernet data packets in particular have a prescribed maximum length, so that if extra data is added to a packet which is close to or equal to the maximum length, the packet will become “illegal”. In existing multi-chip devices such a packet would appear to be in error and may be discarded or truncated by the destination device. In both cases, data will be lost.
Using the preamble to a data packet to convey additional information may be feasible in some circumstances but in general it would require redesign of existing devices.